1.1 Background of the Study

With the increasing use of LED lighting, there is a trend toward simpler semiconductor-based AC-powered direct-driven products, addressing reliability, cost, and size issues present in converter-based LEDs^{[1, 2]}. Despite the interest in AC direct drives, voltage fluctuations lead to flicker problems, resulting in shorter lifetimes of components, including capacitors^{[3, 4]}.

Flicker and electrical properties significantly influence LED performance, size, cost, and applications^[1-4]. The former affects human eyes and health, while the latter, including power factor, total harmonic distortion (THD), and rated power impact efficiency, performance, and safety^{[3, 4]}. Therefore, international standards and measurement methods have been established^[5-8].

Some AC-powered direct-driven lighting exhibit favorable flicker and electrical properties^[2]. However, certain low-cost alternatives using low-level circuits fail to meet basic electrical requirements despite satisfying flicker standards. Others comply with neither flicker nor electrical requirements yet still reach the market.

Various direct AC-driven technologies have been proposed in developing LED lighting fixtures, considering issues, such as SMPS cost and quality. However, as standards on flicker for lighting products become stricter and performance requirements increase, there is a growing demand for products and technologies that exhibit superior flicker characteristics. This is essential not only for general lighting but also expansion into human- centric lighting applications.

Additionally, blinking and cyclical on-off lighting patterns impact light pollution assessments and cause visual discomfort^[9]. High-power home electrical systems, such as induction heaters can induce blinking phenomena, affecting AC-powered direct-driven lighting products.

1.2 Objectives of the Study

The objective of this study is to find the optimal method for AC-powered direct-driven lighting products by examining electrical properties, flicker, and blinking to achieve stable and comfortable lighting. The percent flicker and electrical characteristics of four market-available AC-powered direct-driven LED lighting devices, as well as a proposed drive were measured. Each device uses different technology for AC operation. The relationship between flicker and electrical characteristics was investigated by comparing and analyzing current waveforms. The blinking phenomena of the proposed device caused by electrical inductions were also studied.

2.1 Conventional AC Power Drive

Fig. 1 shows a two-channel AC-powered direct-driven LED circuit. This driving method sequentially operates LEDs in response to the input voltage level. When the input voltage exceeds VLED1, only LED1 is turned on. When the input voltage surpasses VLED1 + VLED2, both LED1 and LED2 are activated.

Fig. 1. Conventional AC power-drive circuit

The device operation is illustrated in Fig. 2(a) presents the current flow in the device and the LEDs that light up according to the input voltage. Fig. 2(b) shows the LEDs that light up in each segment. As there is a segment “0” where all LEDs are in the off state, the percentage flicker is 100, which is an inferior characteristic.

Fig. 2. LED Illumination characteristics of conventional AC power drive

2.2 Low-flicker AC Power Drive

Fig. 3 illustrates a traditional method, the valley-fill circuit, which is used to improve the flicker characteristics of AC-powered direct-driven circuits. When the input voltage is high, the current flows through the red dotted line, charging capacitors C1 and C2 then turning on the LEDs. When the input voltage is low, the current is supplmented by power from the capacitors to the LEDs, as depicted with by the blue solid line, thereby turning on the LEDs. As the capacitors discharge, C1 and C2 operate in parallel, producing voltage that is less than half the input voltage and consequently, lighting up only LED1.

Fig. 3. Low-flicker AC power-drive circuit

As shown in Fig. 4(a), although there is no segment where all LEDs are in the off state, the percent flicker is 33.3% (=(100-50)/(100+50)); in most cases, LED2 has a higher current than LED1, resulting in a flicker characteristic greater than 40%. Although the low-flicker AC power-drive method provides improvement compared to the conventional AC power drive circuit, it does not meet the percentage flicker mandated in the lighting product market (below 30%). Therefore, further improvements in the flicker characteristics are required.

Fig. 4. LED Illumination characteristics of a low-flicker AC power drive

2.3 Proposed Very-low-flicker AC Power Drive

Fig. 5 explains the segment-by-segment operation of the proposed very-low-flicker AC power-drive circuit. The very-low-flicker circuit uses a synchronous-drive IC to control the series and parallel connections of the LED array. When current is supplied to the LEDs from the capacitor C1 and the voltage of the capacitor C1 decreases, the synchronous-drive IC switches the LEDs to parallel, allowing for continued LEDs operatation. Resistors RSET1 and RSET2 control the current magnitude when the LEDs operate in series and parallel, ensuring that the current flowing through the LEDs remains constant. Additionally, a controller is used to constantly control the charging and discharging current of the capacitor to the desired current level, as depicted in the circuit.

Fig. 5. Segment-by-segment operation of the very-low-flicker AC power-drive circuit

In Segment ⓪, as shown in Fig. 5(a), current is supplied from the capacitor C1 to the LEDs. As the current charged in the capacitor C1 is supplied to the LEDs, the voltage of the capacitor decreases. Due to the synchronous drive, even if the voltage drops to LED1, it can still be supplied to LED2, ensuring that the LEDs operate continuously in Segment ⓪. When current I1 flows, the synchronous-drive IC configures LED1 and LED2 in parallel, allowing the currents I1 and I2 to flow simultaneously. In Segment ①, as shown in Fig. 5(b), LED1 and LED2 are arranged in parallel, allowing the same current to flow through both the LEDs simultaneously. In Segment ②, as shown in Fig. 5(c), the LED array is configured in series, allowing the same current to flow through both LED1 and LED2.

Fig. 6. LED Illumination characteristics of a very-low-flicker AC power drive

Fig. 6(a), shows the LED illumination characteristics when the proposed very-low-flicker AC power-drive circuit is in operation. The red dashed line represents the current as viewed from the input side. The controller allows a larger current to flow through the capacitor, while a consistently low current flows through the LEDs. The excess current is stored in the capacitor and supplied to the LEDs when the input voltage decreases. Thus, as shown in Fig. 6(b), consistent LED operation with the same current magnitude across all segments is ensured. By controlling the capacitor, the LED illumination can be adjusted, achieving excellent flicker characteristics with smaller capacitors compared to the traditional low-flicker method.

2.4 Comparison of Characteristics of Different Types of AC Power Drives

Table 1 presents the flicker and electrical characteristics of the linear-bar-type lighting devices using the three types of AC power-drive circuits mentioned previously. The samples used in the experiments are shown in Fig. 7.

Sample 1 is a product that uses a conventional AC power-drive method, showing 100% percent flicker but relatively good electrical characteristics. Sample 2 and Sample 3 use the low-flicker AC power-drive method. Sample 2 shows an improved percent flicker of 35.6% and relatively good electrical characteristics. In contrast, Sample 3 exhibits a flicker of 1.32%, indicating excellent flicker characteristics, but very poor electrical characteristics, requiring caution during daily use. Sample 4 utilizes the proposed very-low-flicker AC power-drive method, introducing 1.9% percent flicker, 24.6% THD, and 96.9% power factor. Compared with Sample 3, Sample 4 has a higher power factor and a lower THD ensuring a similar percent flicker. THD is somewhat higher than that of Sample 2 because the rectangular waveform has higher-order frequencies compared to the sinusoidal shape.

Fig. 7. Samples of linear LED bar modules powered by individual AC direct drives

The current waveforms for the samples are presented in Fig. 8, Sample 1, Sample 2, and Sample 4 display relatively stable current waveforms resembling a sinusoidal shape. However, Sample 3 shows a different waveform with an overshoot in front and descending in the end, and is therefore predicted to be vulnerable to external electrical shocks.

Fig. 8. Current waveform characteristics of the each sample powered by the individual AC direct drive

The relation between flicker and electrical characteristics can be summarized as follows: First, to achieve low flicker values LEDs should always turn on and ripples should be eliminated by adopting a filter circuit. Second, a sinusoidal waveform gives less THD than sawtooth or rectangular shape because the former exhibits fewer high-order frequencies than the latter. Third, decaying waveforms, such as sawtooth forms, should be avoided as they are susceptible to short pulses having a narrower duration than the decaying time. Finally, power factor or efficiency can be decreased by adding more components to provide a lower percent flicker and THD values, sacrificing reliability.

As shown in Fig. 9, when using such AC LED products similar to Sample 3, there is a sharp peak in front of the ripples, providing a risk of damaging bridge diodes because of the rapid inrush of a 4-8A current after initially being powered. Note that the first peak has a duration less than the decay times of the following ripples, so the time constant should be further optimized. Some products operate in a more unstable state when the current from the capacitor is fully discharged, leading to a higher inrush of current and preventing the use of higher capacitance. These products often caution against the use of more than five identical products connected in series and do not support dimmers. Additionally, owing to their high total THD, such lighting products can affect other electrical devices on the same PCB, making them unsuitable for use. Those lighting products that satisfy only flicker characteristics are the most widely sold due to low-cost AC power drives in the domestic lighting market. Most of these products use cheap AC-driver ICs.

Fig. 9. Initial power input waveform for Sample 3 powered by AC power drive

2.5 Analysis of Optical Blinking Phenomenon

To reproduce the optical blinking phenomenon in LED lighting products caused by induction and similar electrical devices, an experiment was conducted using a 2kW induction device. Reproducing the optical blinking phenomenon was not straightforward, and only Sample 1, which exhibited a high percent flicker, showed slightly noticeable blinking. The other samples did not show any significant blinking phenomenon.